Commit message: Prevent copying uninitialized data in `AssignOp`.

This prevents harder to debug undefined behaviors that cannot be traced back to the original tensor after assignments occur earlier in the graph execution. Several of these undefined behaviors are just reference bindings to null pointers, which are caught when running under ubsan/asan.

PiperOrigin-RevId: 408654780
Change-Id: Iad2ec40d43f5fd7ea016c20283356c12d5ddeab1
CVE: CVE-2022-23573
CWE: CWE-908
Vulnerability description: Tensorflow is an Open Source Machine Learning Framework. The implementation of `AssignOp` can result in copying uninitialized data to a new tensor. This later results in undefined behavior. The implementation has a check that the left hand side of the assignment is initialized (to minimize number of allocations), but does not check that the right hand side is also initialized. The fix will be included in TensorFlow 2.8.0. We will also cherrypick this commit on TensorFlow 2.7.1, TensorFlow 2.6.3, and TensorFlow 2.5.3, as these are also affected and still in supported range.

Project: tensorflow
File: assign_op.h

  void Compute(OpKernelContext* context) override {
    const Tensor& rhs = context->input(1);

    // We always return the input ref.
    context->forward_ref_input_to_ref_output(0, 0);

    // We can't always know how this value will be used downstream, so make
    // conservative assumptions in specifying constraints on the memory
    // allocation attributes, unless the Grappler graph analysis determined that
    // it was safe not to.
    AllocatorAttributes attr;
    if (!relax_constraints_) {
      attr.set_gpu_compatible(true);
      attr.set_nic_compatible(true);
    }

    {
      mutex_lock l(*context->input_ref_mutex(0));
      const Tensor& old_lhs = context->mutable_input(0, /* lock_held */ true);
      const bool same_shape = old_lhs.shape().IsSameSize(rhs.shape());
      if (validate_shape_) {
        OP_REQUIRES(context, same_shape,
                    errors::InvalidArgument(
                        "Assign requires shapes of both tensors to match. "
                        "lhs shape= ",
                        old_lhs.shape().DebugString(),
                        " rhs shape= ", rhs.shape().DebugString()));
      }

      // In the code below we try to minimize the amount of memory allocation
      // and copying by trying the following two shortcuts:
      // 1. If the lhs is initialized and has the same number of elements as
      //    the rhs we can avoid a memory allocation.
      // 2. If we can reuse the rhs buffer we avoid both a memory allocation
      //    and copying.

      // 1. Try to copy into an existing buffer.
      if (old_lhs.IsInitialized() &&
          old_lhs.shape().num_elements() == rhs.shape().num_elements()) {
        // The existing lhs tensor has already been initialized and the right
        // hand side can fit in the underlying buffer.
        Tensor reshaped_old_lhs;
        if (same_shape) {
          reshaped_old_lhs = old_lhs;
        } else {
          CHECK(reshaped_old_lhs.CopyFrom(old_lhs, rhs.shape()));
          context->replace_ref_input(0, reshaped_old_lhs,
                                     /* lock_held */ true);
        }
        if (use_exclusive_lock_) {
          Copy(context, &reshaped_old_lhs, rhs);
          return;
        }
      } else {
        // 2. Try to reuse the rhs.
        std::unique_ptr<Tensor> input_alias = context->forward_input(
            1, OpKernelContext::Params::kNoReservation /*output_index*/,
            rhs.dtype(), rhs.shape(), DEVICE_MEMORY, attr);
        if (input_alias != nullptr) {
          // Update the ref to point to the new buffer.
          context->replace_ref_input(0, *input_alias, /* lock_held */ true);
          return;
        }

        // Otherwise, create a new tensor whose shape matches the
        // right hand side, hand off to lhs and copy the rhs into it.
        Tensor copy_tensor;
        OP_REQUIRES_OK(context,
                       context->allocate_temp(old_lhs.dtype(), rhs.shape(),
                                              &copy_tensor, attr));
        // We track memory of variables in variable ops instead of in this
        // assign op.
        context->clear_recorded_memory();
        context->replace_ref_input(0, copy_tensor, /* lock_held */ true);
        if (use_exclusive_lock_) {
          Copy(context, &copy_tensor, rhs);
          return;
        }
      }
    }

    // The tensor has already been initialized and the right hand side
    // matches the left hand side's shape. We have been told to do the
    // copy outside the lock.
    Tensor old_unlocked_lhs = context->mutable_input(0, /* lock_held */ false);
    Copy(context, &old_unlocked_lhs, rhs);
  }